The present invention relates generally to alloy compositions having a tri-nickel aluminide base. More specifically, it relates to rapidly solidified tri-nickel aluminide base materials which include quantities of strengthening and ductilizing additives and which may be processed into useful articles. Also it relates to rapidly solidified tri-nickel aluminide base alloy which has improved properties based on a combination of doping and alloying and working.
It is known that polycrystalline tri-nickel aluminide castings exhibit properties of extreme brittleness, low strength and poor ductility at room temperature.
The single crystal tri-nickel aluminide in certain orientations does display a favorable combination of properties at room temperature including significant ductility. However, the polycrystalline material which is conventionally formed by known processes does not display the desirable properties of the single crystal material and, although potentially useful as a high temperature structural material, has not found extensive use in this application because of the poor properties of the material at room temperature.
It is known that nickel aluminide has good physical properties at temperatures above 1000.degree. F. and could be employed, for example, in jet engines as component parts at operating or higher temperatures. However, if the material does not have favorable properties at room temperature and below the part formed of the aluminide may break when subjected to stress at the lower temperatures at which the part would be maintained prior to starting the engine and prior to operating the engine at the higher temperatures.
Alloys having a tri-nickel aluminide base are among the group of alloys known as heat-resisting alloys or superalloys. These alloys are intended for very high temperature service where relatively high stresses such as tensile, thermal, vibratory and shock are encountered and where oxidation resistance is frequently required.
Accordingly, what has been sought in the field of superalloys is an alloy composition which displays favorable stress resistant properties not only at the elevated temperatures at which it may be used, as for example in a jet engine, but also a practical and desirable and useful set of properties at the lower temperatures to which the engine is subjected in storage and mounting and starting operations. For example, it is well known that an engine may be subjected to subfreezing temperatures while standing on an airfield or runway prior to starting the engine.
Significant efforts have been made toward producing a tri-nickel aluminide and similar superalloys which may be useful over such a wide range of temperature and adapted to withstand the stress to which the articles made from the material may be subjected in normal operations over such a wide range of temperatures.
For example, U.S. Pat. No. 4,478,791, assigned to the same assignee as the subject application, teaches a method by which a significant measure of ductility can be imparted to a tri-nickel aluminide base metal at room temperature to overcome the brittleness of this material.
Also, copending applications of the same inventors as the subject application, Ser. Nos. 647,326; 647,327; 647,328; 646,877 and 646,879 filed Sept. 4, 1984 teach methods by which the composition and methods of U.S. Pat. No. 4,478,791 may be further improved. These applications and this patent are incorporated herein by reference.
For the unmodified binary intermetallic, there are many reports in the literature of a strong dependence of strength and hardness on compositional deviations from stoichiometry. E.M. Grala in "Mechanical Properties of Intermetallic Compounds", Ed. J.H. Westbrook, John Wiley, New York (1960) p. 358, found a significant improvement in the room temperature yield and tensile strength in going from the stoichiometric compound to an aluminum-rich alloy. Using hot hardness testing on a wider range of aluminum compositions, Guard and Westbrook found that at low homologous temperatures, the hardness reached a minimum near the stoichiometric composition, while at high homologous temperature the hardness peaked at the 3:1 Ni:Al ratio. Trans. TMS-AIME 215 (1959) 807. Compression tests conducted by Lopez and Hancock confirmed these trends and also showed that the effect is much stronger for Al-rich deviations than for Ni-rich deviations from stoichiometry. Phys. Stat. Sol. A2 (1970) 469. A review by Rawlings and Staton-Bevan concluded that in comparison with Ni-rich stoichiometric deviations, Al-rich deviations increase not only the ambient temperature flow stress to a greater extent, but also that the yield stress-temperature gradient is greater. J. Mat. Sci. 10 (1975) 505. Extensive studies by Aoki and Izumi report similar trends. Phys. Stat. Sol. A32 (1975) 657 and Phys. Stat. Sol. A38 (1976) 587. Similar studies by Noguchi, Oya and Suzuka also reported similar trends. Met. Trans. 12A (1981) 1647.
More recently, an article by C.T. Liu, C.L. White, C.C. Koch and E.H. Lee appearing in the "Proceedings of the Electrochemical Society on High Temperature Materials", ed. Marvin Cubicciotti, Vol. 83-7, Electrochemical Society, Inc. (1983) p. 32, discloses that the boron induced ductilization of the same alloy system is successful only for aluminum lean Ni.sub.3 Al.
The tri-nickel aluminide alloys known in the prior art display a positive strength relationship to temperature. That is the strength of these aluminides increases as the temperature is increased. Such prior art alloys are known to be stronger at 600.degree. C. than they are at room temperature. What is desirable and is sought in relation to such alloys is a more rapid increase in strength with increasing temperature.
Also it is known that a ductile tri-nickel aluminide will undergo a moderate degree of work hardening. For example if a specimen of a boron doped and moderately ductile tri-nickel aluminide is rolled to reduce its thickness by about 10% the specimen is made harder by this rolling. What is known to be desirable and to be sought in relation to such aluminides is a composition which will undergo greater hardening for a given extent of working, as for example a 10% working. An alloy which undergoes greater work hardening at all degrees of working or degrees of strain, i.e. an alloy which undergoes greater strain hardening over the entire strain range, is highly preferable.
Moreover what is particularly desirable is a material which displays a higher strain hardening rate and which displays the higher strain hardening rate over the entire strain range.
The subject application presents a further improvement in the nickel aluminide to which significant increased ductilization has been imparted.